Method of identifying a gas

ABSTRACT

A method and apparatus for identifying an unknown reactive gas in a carrier gas, utilizing a sensor with a diffusion limited inlet. After a signal is established for the carrier gas, a flow of the mixture of carrier gas and reactive gas is passed to the sensor and a steady state signal S is established. Then, the input to and output from the sensor are closed, and the steady state signal decays as a known volume of reactive gas is consumed. The decay curve of the signal is integrated to produce an integrated response Σ, and the ratio S/Σ is proportional to the diffusion coefficient for the reactive gas. By comparing this ratio to the ratio for a known reactive gas, the identity of the unknown reactive gas can be determined.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a method and apparatus for identifying acomponent of gas mixture.

2. Description of Related Art

Potentially dangerous gas mixtures may be found in many workplaceenvironments, the dangers including the risk of fire or explosion fromcombustible gases, oxygen enrichment or deficiency and exposure to toxicgases. These dangers are well known and gas detection instruments areavailable to detect a wide range of gases. These instruments typicallycontain one or more gas sensors, which give a proportional electricalresponse dependent upon the concentration of the gas to be detected. Ifthe concentration exceeds allowed concentration limits, then theinstrument will provide an alarm to warn nearby personnel, or it mayactivate other remedial action, such as increasing the ventilation. Gasdetection instruments for safety applications are broadly divided intotwo groups, portable instruments, which are designed to be hand held orworn by the user and provide personal monitoring, and fixed instruments,typically wall mounted, which provide area monitoring.

Combustible gases are often characterized by their lower explosivelimit, (LEL), which is the minimum concentration of that particular gasin air, which can support combustion. If the concentration is below theLEL, then the gas will not burn without the continued support of anexternal ignition source. If the concentration of the gas is greaterthan the LEL, then once ignited, the combustible gas--air mixture willburn, without the need for an external heat source. Indeed, manycombustible gas--air mixtures will explode if ignited at concentrationsgreater than the LEL. At very high concentrations of the combustiblegas, there may be insufficient oxygen to support the combustion, and thecombustible gas--air mixture will no longer burn. This upperconcentration limit for flammability is known as the upper explosivelimit (UEL). The upper and lower explosive limits depend on the gas tobe detected, as may be seen from the following data, taken from the CRCHandbook of Chemistry and Physics, 68^(th) Edition, 1987-1988, CRCPress, Boca Raton, Fla.

    ______________________________________                                        Limits of Flammability of Gases in Air                                        Gas                  LEL (% vol.)                                                                             UEL (% vol.)                                  ______________________________________                                        Acetylene                                                                              HCCH        2.50       80.00                                         Ammonia  NH3         15.5       27.0                                          Benzene  C.sub.6 H.sub.6                                                                           1.40       7.10                                          Hydrogen H.sub.2     4.00       74.24                                         Methane  CH.sub.4    5.00       15.00                                         Pentane  C.sub.5 H.sub.12                                                                          1.40       7.80                                          Xylene   C.sub.6 H.sub.4 (CH.sub.3).sub.2                                                          1.00       6.00                                          ______________________________________                                    

There are three main types of sensors used to detect combustible gases.For general leak detection, metal oxide, especially tin oxide sensorsare used. The electrical conductivity of the metal oxide changes whenexposed to the combustible gas at high temperatures. These sensors arerarely used for safety monitoring, since they commonly lack theprecision necessary for this application.

Infrared sensors typically measure the absorption of the gas at 2940cm⁻¹ (˜3.4 μm), which corresponds to the carbon-hydrogen (C--H) bondstretching frequency. The absorption of the infrared light depends onthe number of C--H bonds stretching in the molecule. One of thelimitations of infrared detectors is that molecules such as carbonmonoxide (CO) and hydrogen (H₂) do not have an absorbance at or near 3.4μm bond, since they do not have any C--H bonds. Even molecules such asacetylene (HCCH) and benzene (C₆ H₆) which both have C--H bonds oftenhave low sensitivity at 3.4 μm since the triple bond in acetylene andthe aromatic ring in benzene shift the absorbance of the C--H stretch tohigher frequency. These effects of molecular substitution on the C--Hbond vibration frequency are well known, and can be found in standardtexts such as D. H. Williams, I. Fleming, "Spectroscopic methods inOrganic Chemistry", third Edition, McGraw-Hill book Company, Ltd.,London, 1980.

The other major type of sensor for combustible gas is the catalytic beadsensor, which measures heat of combustion. The detector bead of acatalytic bead sensor comprises a small platinum coil encased in aceramic bead containing precious metal catalysts. The combustible gasenters the sensor and travels to the catalytic bead by naturaldiffusion. The gas is combusted at the bead surface, aided by thecatalysts and the resulting release of heat raises the temperature ofthe bead. This rise in temperature results in an increase in resistanceof the platinum coil, which is normally detected using a Wheatstonebridge. Within the sensor, there is usually a second bead, the referenceor compensator bead, which is constructed similarly to the detectorbead, without the catalyst. The compensator bead comprises one of theother arms of the Wheatstone bridge, and it is used to cancel out anyother non-combustion related responses of the beads, such as changes inambient humidity or thermal conductivity of the gas. The response of thecatalytic bead depends primarily on the heat of combustion of the gasand the rate at which the gas can diffuse to the detector bead.

It is common practice to express the concentration of combustible gasesas a percentage of the LEL, and thus 2.5% by volume of methane is 50%LEL. The response of catalytic bead sensors is approximately linear overtheir useful range (0 to 100% LEL), and setting the empiricallydetermined proportionality constant between the output response and theconcentration is called calibration. However, the sensitivity to gasvaries with the type of gas. Compared to a relative response to 50% LELof methane of 1.0, the response to 50% LEL pentane is only about 0.5. Amore thorough discussion of catalytic bead sensors may be found in thereview by J. G. Firth, "Measurement of Flammable Gases and Vapors" in C.F. Cullis, J. G. Firth (Eds.), "Detection and Measurement of HazardousGases", Heinemann, London, 1981.

Many of the commonly encountered toxic gases are detected usingamperometric electrochemical gas sensors. A typical electrochemicalsensor is usually constructed with two or more electrodes in contactwith an electrolyte. The electrode is usually separated from the outsideenvironment by a gas porous membrane, and other diffusion barriers. Thegas to be detected enters the sensor and passes through the membrane tothe working electrode, where is it either oxidized or reduced;alternatively, the rate of oxidation or reduction of the electrode oranother species in electrolyte may be limited by the availability of thetoxic gas. The resulting electrical current is proportional to the rateat which the gas is being consumed by the electrode. The output currentis therefore usually linearly proportional to the gas concentration,since the response is limited by the rate at which the gas to bedetected can diffuse into the sensor.

The nature of the response of the sensor to a toxic gas depends on boththe design of the sensor and the nature of the gas. Some gases such ascarbon monoxide (CO) and hydrogen (H₂) are oxidized at the electrode,whereas other gases such as chlorine and nitrogen dioxide are usuallyreduced at the sensor electrode. While the oxidation of carbon monoxideto carbon dioxide (CO₂) is a two-electron process, the oxidation ofhydrogen sulfide (H₂ S) to sulfuric acid (H₂ SO₄) is an eight-electronprocess. Thus, a diffusion limited sensor which responds to bothhydrogen sulfide and carbon monoxide will give a stronger response tothe hydrogen sulfide, for a given concentration of gas.

The above examples of sensor technology are intended to illustrate thatthe signal obtained for a combustible or toxic gas depends on both thesensor technology employed, and on the properties of the individualgases. This fact poses a quandary for personnel who risk being exposedto a variety of different gases. If they use a broad band sensor, i.e. asensor that is sensitive to a wide variety of gas types, then there isthe risk that the alarm levels will not be appropriate for any givengas. However, if they instead decide to use sensors selective for aparticular gas, then there is the risk that if an unanticipatedhazardous gas is present, then it may not be detected at all. Inaddition, they may have to use instruments which contain many sensors orthey may have to carry several instruments, which may be both expensiveand cumbersome.

It is now common practice to use a broad band sensor for combustiblegases, such as a sensor based on infrared or catalytic bead technology,and to set the alarm levels to match the gas with the least sensitiveresponse. If there is likely to be either methane or pentane in aparticular environment, then a catalytic sensor based instrument willusually be calibrated with pentane, since pentane has the lowersensitivity. However, this approach can result in false alarms since asafe concentration of methane will set the instrument into alarm. With abroad band sensor, such as the catalytic bead sensor, it is notcurrently possible to distinguish whether a response is coming frommethane or pentane.

In contrast, toxic gases are usually detected with sensors specific to aparticular gas. This difference between the combustible gases and thetoxic gases is in part due to the wide variation in risk associated witha toxic gas. For example, carbon dioxide has an OSHA eight-hourpermissible exposure limit (PEL) of 5000 ppm, carbon monoxide has a PELof 50 ppm, sulfur dioxide has a PEL of 5 ppm and chlorine dioxide has aPEL of 0.1 ppm. Another reason for this difference is that it is ofteneasier to fabricate an electrochemical sensor to be selective to aparticular toxic gas than to fabricate a catalytic bead sensor to beselective to a particular combustible gas. Some attempts have been madeto produce broad band electrochemical gas sensors, but they also sufferfrom the drawback of deciding where to set the alarm levels. Forexample, a sensor which gives a response to both sulfur dioxide andcarbon monoxide, corresponding to 10 ppm, is five times below the OSHAPEL for carbon monoxide but twice the OSHA PEL for sulfur dioxide, eventhough both gases are oxidized by two electrons.

Clearly, a method would be desirable that enables a gas detectioninstrument to identify the gas, and then select the appropriate alarmthreshold levels to be used.

Several methods have been developed to identify the components of apotentially hazardous atmosphere. Detector tubes kits are available, inwhich a series of colorimetric tubes are used, initially identifying theunknown gas by broad chemical classification (e.g. acidic, halogenatedor reducing gas), followed by successive iterations until the gas isidentified. However, this manual approach is time consuming, cumbersome,and provides the analysis only at a single moment in time ("Drager-TubeHandbook," 8^(th) Edition, National Draeger Inc, Pittsburgh, Pa.).

In the past, gases were identified by collecting a sample, either on anabsorbent, such as activated charcoal, or in a clean gas chamber,followed by laboratory analysis. This method is time consuming and sincethe analysis has to be performed elsewhere, there is often aconsiderable delay from the time the sample is taken to the time whenthe identity of the gas is determined.

Laboratory based methods for gas identification usually involve large,expensive and immobile equipment, such as mass spectrometers, gaschromatographs and infrared spectrometers. Considerable effort has beenmade to adapt these laboratory instruments for use in the field, andseveral manufacturers do offer portable gas chromatographs (for exampleHNU Systems, Newton Mass. 02461 and Viking Instruments Corporation,Chantilly, Va. 20151). Mass spectrometers offer high sensitivity andgood selectivity, and despite the difficulties of requiring a highvacuum and other engineering challenges, portable and semi-portable massspectrometers have been developed, by several companies. For exampleFoster-Monitor Group, of Cheswick Pa., has a mass spectrometer that cancollect a sample and identify the component gases based on theirmolecular masses. Both the portable mass spectrometers and gaschromatographs are capable of identifying an unknown gas. While thesedevices offer considerable potential, they remain too expensive forroutine safety monitoring, and tend to be used for more specializedapplications.

Infra-red spectroscopy can also be used to identify a particular gas.Whereas most infrared combustible gas sensors operate at a singlewavelength, and thus have difficulty distinguishing between varioushydrocarbons, the full infrared spectra of most organic compounds areunique. Thus, if the full spectrum is obtained with an infraredspectrometer, then the spectrum can be compared against a library ofinfrared spectra. Due to the complexity of the optics, the costassociated with obtaining a full spectrum and the subsequent dataanalysis increases the cost of this instrument well beyond that normallyused for routine safety monitoring.

Sensor arrays have been developed which are capable of identifying awide range of gases. These sensor arrays effectively have a large numberof sensor elements, each with a different response characteristic. Thecombined pattern of response from a sensor array can indicate theconcentration and identify of gaseous species present. However, sensorarrays have two drawbacks. The first drawback is that the patternrecognition requires very complex mathematical analysis, and thus theyrequire a significant computer analysis to achieve useful results. Thesecond and more important drawback is that the present day sensor arraysdo not have the reliability necessary for safety applications. It islikely that both of these problems will be overcome in the future, andsensor arrays are now commercially available, for example from CyranoSciences Inc, Pasadena, Calif., and are being used for applications suchas food quality and wine classification. The operation of these sensorarrays has been described by M. S. Freund and N. S. Lewis in proceedingsof the National Academy of Science (1995), 92, 2652-2656 and by N. S.Lewis in U.S. Pat. No. 5,571,401.

As may be seen from the above discussion, there is a need for aneconomical method that will identify an unknown gas, so that theappropriate calibration and alarm set point values can be selected. Theinstrument should be economical enough to be incorporated in personneland fixed-point safety monitoring equipment. Furthermore, the accuracyand precision of the gas concentration measurement should be as good asthe present technology, and preferably the instrument should still usethe existing sensor technologies, since they are well tested and have agood service record.

Recently, Zdanevitch et al in U.S. Pat. No. 5,709,792 described a methodfor identifying a combustible gas using a catalytic bead sensor byvarying the potential applied to the bead and using the differences intemperature needed to initiate combustion as a means to identify thegas. This method has many advantageous features, but it suffers from thelimitation that the temperature of combustion may vary as the sensorcatalyst ages or if the sensor is exposed to poisoning or inhibitingcompounds. This deleterious effect will be much more pronounced than isnormally observed with a conventional sensor operating under diffusioncontrol at a temperature sufficiently high to oxidize all combustiblegases, which reach the detector element of a catalytic bead sensor.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the invention to provide a method foridentifying an unknown reactive gas which overcomes the disadvantages ofprior art methods.

It is a further object of the invention to provide a method fordetermining concentration of a gas identified by the method of theinvention.

It is still a further object of the invention to use the measurementsmade according to the invention to determine whether the gas detectioninstrument is operating correctly.

To achieve these and other objects, the invention is directed to amethod and apparatus for identifying a gas utilizing a known reactivegas detector including a diffusion barrier inlet which generates anelectrical signal upon exposure to the gas. A relative diffusionconstant is determined for an unknown reactive gas by comparison of asteady state signal for the unknown gas with the integrated decay signalfor the detector exposed to a sealed and predetermined volume of thegas. The relative diffusion coefficient can be compared with therelative diffusion coefficient of a standard gas determined in a similarmanner and the resulting actual diffusion coefficient is used toidentify the gas.

Once the gas is identified, the concentration can be determined from thevalue of the steady state signal for the gas.

The invention includes an apparatus for carrying out the method of theinvention, including a detector of known type with diffusion barrierinlet, a manifold through which the gas passes and to which the detectoris exposed through the diffusion barrier and means for closing the inletand outlet of the manifold so that the detector is exposed to a fixedvolume of gas. The apparatus can include computer means for automatingthe steps of the invention to any degree desired.

The method of the invention can be used to detect a wide variety ofreactive gases. Among the gases which can be detected are combustiblegases such as alkanes, alkenes, alkynes, alcohols, ethers, esters andaromatics, and toxic gases such as carbon monoxide, sulfur dioxide,hydrogen sulfide, nitric oxide, nitrogen dioxide and chlorine. The abovelist is representative only, and should not be considered limiting.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of a prior art electrochemical gassensor;

FIG. 2 is a schematic representation of a prior art catalytic bead gassensor;

FIG. 3 is a schematic diagram of a gas analysis system according to theinvention;

FIG. 4 is a schematic diagram of an alternative gas analysis systemaccording to the invention; and

FIG. 5 is a schematic diagram of a further alternative gas analysissystem according to the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A typical electrochemical sensor 10 is shown in FIG. 1. The sensorcomprises a sensor body 11, containing three electrodes, the workingelectrode 12, the reference electrode 13 and the counter electrode 14.The three electrodes 12, 13, and 14 are separated by inert media 15,soaked in electrolyte. Typically this inert media 15 is comprised ofglass paper, and the electrolyte may be an aqueous or non-aqueoussolution of a salt or acid, to provide ionic electrical conductivitybetween the electrodes 12, 13 and 14. The electrolyte is retained withinthe sensor 10 and is prevented from entering the gas volume 16 by acompression of an O-ring seal 17, and the electrodes 12, 13 and 14 andthe sensor housing 11. The gas to be detected diffuses to the sensor 10and enters the gas entry hole 18 into a volume 16 within the sensor 10.The gas diffuses through the working electrode membrane 12, whichtypically comprises a porous membrane with a precious metal (not shown)fixed onto the inner surface of the membrane comprising electrode 12.The magnitude of the steady state response of most amperometricelectrochemical gas sensors is limited by the rate at which the gas tobe detected can diffuse into the sensor. The sensors are designed to belimited by the rate of gas diffusion by making the gas entry hole 18small enough that it presents a significant diffusion barrier to thegas. The advantages of making the sensor diffusion limited are that theresponse is linear with concentration and the sensitivity of the sensor(sensitivity=steady state response to the gas/gas concentration) becomesindependent of small variations in electrode potentials, or small lossesin electrode catalytic activity.

The operation of this electrochemical sensor has been described forillustrative purposes only and many variations on electrochemical sensordesign are known in the prior art. Further details of electrochemicalsensor operation and design may be found in the following references: S.C. Chang, J. R. Stetter, C. S. Cha, "Amperometric Gas Sensors", Talanta(1993), 40 (4) 461-477; B. S. Hobbs, A. D. S. Tantram, R. Chan-Henry in"Techniques and Mechanisms in Gas Sensing", Ed. P. T. Moseley, J. O. W.Norris and D. E. Williams, Adam Hilger , Bristol, (1991).

A catalytic bead sensor 20, as shown in FIG. 2 usually comprises twobeads, a detector 21 and a reference bead 22, the beads comprising awire coil embedded in alumina or other ceramic. The ends 23 of this wireare attached to posts 24 in such a way as to suspend the beads 21 and 22between the posts 24. These posts 24 serve the dual role of mechanicallysupporting the beads 21 and 22, and also providing an electricalconnection to external contacts 25. The two beads 21 are 22 are usuallyseparated by a partition 26, which serves to reduce air currents withinthe sensor 20.

The detector bead 21 additionally has a precious metal catalyst 21adeposited on the surface of the bead 21, whereas the reference bead 22does not. The beads 21 and 22 are held at a high temperature, typicallygreater than 500° C., which in combination with the catalyst facilitatesthe combustion of any combustible gases which reach the detector bead21. Since the reference bead 22 does not have the catalyst on itssurface, no combustion occurs on the reference bead 22. The increase intemperature from the combustion of the combustible gas on the detectorbead 21 results in an increase in the resistance of the wire coil insidethe bead 21. This small increase in resistance is conveniently detectedwith a Wheatstone bridge circuit (not shown), and provides the outputsignal. The gas enters the sensor 20 by diffusing through a sinteredmetal disk 27, which serves the role of both a diffusion barrier andflame arrestor. Catalytic bead sensors are well known in the prior art,and details may be found in reference texts. (J. G. Firth, "Measurementof Flammable Gases and Vapors" in C. F. Cullis, J. G. Firth (Eds.),"Detection and Measurement of Hazardous Gases", Publ. Heinemann, London,1981).

In common practice, sensors for gas detection are either perated byexposing the sensor to the ambient atmosphere and thus allowing gas todiffuse into the sensor, or the gas detection instrument may include apump which draws in the gas from the ambient atmosphere and then passesit over the sensor. The response of the sensor in an aspirated samplegas detection instrument will still be diffusion limited, since the rateat which the gas enters the sensor will still be limited by diffusion.

FIG. 3 shows the gas flow system of a first embodiment of a gasdetection instrument according to the invention. The gas is drawn infrom the ambient atmosphere by the pump 30 and passes through tubing 31to a two way valve 35. If the valve is activated, then the gas passesthrough valve 35 into a sensor manifold 32, whereupon the gas diffusesinto a sensor 33 of a known type, which gives an output response. Thesensor 33 makes an airtight seal with the sensor manifold 32, by meansof an O-ring seal 34. The gas passes out of the sensor manifold 32 viavalve 36 to an exhaust 37. Valve 36 is open whenever valve 35 isactivated and valve 36 is closed whenever valve 35 is not activated.When valve 35 is not activated, the gas from pump 30 is passed to anexhaust 38. The steady state signal from sensor 33 is obtained by havingvalves 35 and 36 both open.

Consider initially an example in which sensor 33 is an electrochemicalgas sensor. For an electrochemical sensor operating under diffusioncontrol, the steady state current I_(ss) (A) is described by Faraday'slaw

    I.sub.ss =nFφ

where n is the number of electrons, F is the Faraday's constant(9.648×10⁴ C/mol) and φ is the flux of gas (mol/s) reaching theelectrode 12 in FIG. 1.

The flux of gas reaching the electrode 12 is described by Fick's firstlaw of diffusion,

    φ=-DCΔ

where D is the diffusion coefficient of the gas (cm² /s), C_(o) is thebulk concentration of the gas (mol/cm³) and Δ is a parameter whichdescribes the diffusion path of the sensor (cm). The minus sign isusually included in this expression to indicate that the flux of gas isfrom high concentration to low concentration, but the minus sign will beleft out from hereon in for simplicity. These two equations can becombined to give the well know expression for the steady state responseI_(ss) of an electrochemical gas sensor.

    I.sub.ss =nFCDΔ

Assume that there is a gas to which the sensor 33 responds in the airdrawn into the instrument by pump 30, and the sensor response hasreached steady state. If the valves 35 and 36 are then closed, thesensor 33 will consume all of the gas within the volume defined by thetwo valves 35 and 36, the sensor 33 and the sensor manifold 32. Theoutput current from the sensor 33 will decrease with time, and the totalcharge Q which passes, is described by the following equation.

    Q=nFCV

Where V (cm³) is the volume of the gas, which is sealed off by the twovalves 35 and 36 and the other terms are as defined earlier. The ratioof the steady state current prior to closing the valves 35 and 36 to thecharge passed can be calculated as:

    I.sub.ss /Q=(Δ/V)D

Note that the right hand side of this expression is comprised of threeconstants. The first two, Δ and V, are constants of the sensor and thegas detection instrument, and independent of the gas. The third constantD, the diffusion coefficient, is a property of the gas being analyzed.Even though the magnitude of response signal depends on the gasconcentration, and the number of electrons, these other factors cancelout in calculating I_(ss) /Q, leaving D as the only gas dependentvariable.

A similar result is obtained if sensor 33 in FIG. 3 is a catalytic beadsensor. The response of a catalytic bead sensor whose response islimited by the rate of gas diffusion can be described by the followingequation, which is directly analogous to the respective equation for theelectrochemical sensor.

    S=KHDΔC

where S is the steady state output signal (mV), C is the bulkconcentration, H is a gas specific response constant which varies withthe gas, K is a constant of the sensor which is the same for all gasesand D and Δ are the gas diffusion coefficient and a value for thediffusion path of the sensor respectively. If the sensor 33 is placed inthe gas flow system shown FIG. 3, the sensor will give a response as acombustible gas is drawn into the instrument by pump 30, provided thevalves 35 and 36 are open. If the valves 35 and 36 are closed after theoutput of sensor 33 has reached steady state S, the output from thesensor 33 will decrease with time. The integrated response of the sensorS over the time it takes for the sensor output to decrease to zero (theresponse of the sensor in the absence of a combustible gas) is given bythe following equation:

    Σ=KHCV

If the ratio between the steady state response S and the integratedresponse Σ is calculated, the answer is proportional to the gasdiffusion coefficient D.

    S/Σ=(Δ/V)D

Note that as with the electrochemical sensor above, the ratio Δ/V isdependent on the physical properties of the gas detection instrument andon the sensor and independent of the gas, whereas the diffusioncoefficient D is a property of the gas. Note also that the gasconcentration and the response constants K and H have also cancelled outof the expression for S/Σ.

If the sensor is a broad band sensor, which responds to a wide range ofdifferent gas types, then this calculation of the diffusion coefficientD can be used to identify the gas. While in principle the diffusion pathparameter Δ and the volume V can be measured, thus allowing the directcalculation of the gas diffusion coefficient, it is more convenient tomeasure the gas diffusion coefficient relative to a calibration gas. Thegas detection instrument would be exposed to gas of known compositionand known diffusion coefficient, which would provide a calibration ofthe system. Then comparison of the ratio of steady state signal tointegrated signal passed (S/Σ or I/Q) for an unknown gas, can be used tomeasure the diffusion coefficient of that gas and thus provideidentification of the unknown gas.

The diffusion coefficient depends on the molecular weight of both thecomponent gas and the balance gas, and on the molecular size. From apractical standpoint, two similarly sized molecules, for exampleethylene (CH₂ ═CH₂) and ethane (CH₃ --CH₃) are likely to have similardiffusion coefficients and so it would be more difficult to distinguishbetween them. Similarly, the two toxic gases nitric oxide (NO) andcarbon monoxide (CO) will have similar diffusion coefficients, and so itwould be more difficult to distinguish between them using theirdiffusion coefficients. However, two dissimilar gases, such as methane(CH₄) and pentane (C₅ H₁₂), or carbon monoxide and sulfur dioxide (SO₂)will have very different diffusion coefficients, and thus can be morereadily distinguished. The ability to distinguish between two differentgases will depend on the magnitude of their relative diffusioncoefficients and the experimental error in the measurement of thediffusion coefficients.

Once the gas diffusion coefficient has been measured, the gas can beidentified by comparison of the diffusion coefficient with values forknown gases. Once the gas has been identified, then the correctsensitivity parameters, and alarm threshold limits can be used by thegas detection instrument. Most modern gas detection instruments containa microprocessor or other controller, and thus it is relatively easy fora gas detection instrument to do the requisite calculations and toaccess `look-up` tables within the instrument to compare measured withexpected diffusion coefficients and to obtain the sensitivity parametersand the alarm threshold limits.

This method can be used with any type of sensor for which the steadystate response is limited by the rate of gas diffusion into the sensor.Furthermore, it is thus possible to identify a gas with a single sensor,and this gas identification may be performed using sensors of well knownand well trusted design.

Given that the gas can be identified from the steady state response Sand the integrated response Σ, computer means can be added to theapparatus to automate the identification to the extent desired. As shownin FIG. 3, a central processing unit 40 controls the operation of adevice 41 which controls the opening and closing of valves 35 and 36. Adevice 42 contains the electronic circuitry necessary to operate sensor33 and to obtain output readings from sensor 33. Such circuits are wellknown in the art, and include the Wheatstone bridge circuit used inconjunction with a catalytic bead sensor and a potentiostat used inconjunction with an electrochemical sensor. Device 42 may display thereadings on a display unit 43, as well as pass the readings on tocentral processing unit 40. If the unit 40 senses a hazardous condition,an alarm unit 44 is notified.

Typically, central processing unit 40 will store the steady state signalfor the background atmosphere or carrier gas. When the presence of areactive gas is determined, unit 40 will permit a steady state signal Sto be established (S is the signal in the presence of the reactive gasminus the background signal in the absence of the reactive gas), thennotify device 41 to close valves 35 and 36. Unit 40 then records thedecay of the output signal from device 42 and when the output signalreaches the signal for the background, valves 35 and 36 are opened, andmonitoring continues. Meanwhile, unit 40 integrates the decay curve toobtain Σ and uses Σ and S to obtain a diffusion coefficient which can becompared to a list of known coefficients to identify the gas. Once thegas is identified, the response factors and the alarm trigger levels forthat gas can be obtained from a list of known values. Using theseparameters, unit 40 determines the gas concentration from the steadystate signal S, and if concentration exceeds a predetermined level, thealarm unit 44 signals the operator.

EXAMPLE 1

A simple apparatus, shown in FIG. 4, was constructed to demonstrate thepractical utility of this invention. A catalytic bead sensor 50 fromCity Technology, Portsmouth, England, model 4P-100 Cipel, was sealedinto a short length of tubing 51 with a gas tight sealant 52 behind athree quarter-inch diameter ball valve 53. The ball valve 53 isconnected via a short length of tubing 54 to a T connection to a tube 55through which the test gas flows. A small baffle 56 was inserted intothe tube 54 to promote the flow of gas towards the ball valve 53 andsensor 50. When the ball valve 53 is open, the test gas flows towardsthe sensor 50. When the ball valve 53 is closed, the flowing gas isprevented from reaching the sensor 50, and the sensor is exposed only tothe gas in the sealed volume 57 determined by the sensor 50, sealant 52,tubing 57 and the ball valve 53. The sensor 50 was connected to aWheatstone bridge detection circuit (not shown) via leads 58, and theoutput signal was collected on an automated data acquisition system(also not shown). With the ball valve 53 open, the sensor 50 was exposedto the test gas and the output allowed to reach steady state. Oncesteady state had been reached, the ball valve 53 was closed using handle59, and the output from the sensor 50 was followed with time as itdecayed back to the zero point, i.e. approximately the same signal aswas observed with clean air, which was free of combustible gas. Thesignal from the time when the ball valve 53 was closed to the time whenthe output signal had decayed to zero was integrated using a spreadsheetprogram, and the ratio of the steady state signal to the integratedsignal (S/Σ)was calculated.

The apparatus was calibrated using 40% LEL methane in air from acompressed gas cylinder. The steady state signal obtained from sensor 50in a Wheatstone bridge circuit, when the ball valve 53 was open was 47.7mV. When the ball valve 53 was closed, the signal decayed to zero. Thearea under the signal-time graph for the decay was calculated using aMicrosoft Excel spreadsheet, and it was found to be 977 mVs. The ratioof the steady state signal to the integrated decay signal wascalculated, S/Σ=0.048 s⁻¹.

The equipment was then tested with a test gas A, and the steady statesignal for test gas A was 50.2 mV and the integrated decay signal was312 mVs. Therefore the ratio of the steady state signal to theintegrated decay for test gas A 0.16 s⁻¹. The S/Σ ratio for test gas Ais 0.16/0.048=3.3 times larger than the S/Σ ratio for methane, and thusthe diffusion coefficient for test gas A is 3.3 times that of methane.

Diffusion coefficients relative to the calibration gas methane, areshown below. Experimental values of the diffusion coefficient have beentabulated for many gases in the literature, or they can be calculatedusing standard methods (G. O. Nelson, "Gas Mixtures, Preparation andControl", Lewis Publishers, Boca Raton, Fla., 1992; E. L. Cussler"Diffusion, Mass Transfer in Fluid Systems", Cambridge University Press,Cambridge, 1992 ).

    ______________________________________                                        GAS         Diffusion Coefficient (Relative to Methane)                       ______________________________________                                        Hydrogen    3.32                                                              Methane     1.0 (fixed)                                                       Butane      0.49                                                              Pentane     0.39                                                              ______________________________________                                    

Thus, by comparison of the experimental ratio of S/Σ for test gas A tothe ratio S/Σ for methane, with the relative diffusion coefficients inthe table, test gas A is identified as hydrogen.

The experiment was repeated with test gas B. For test gas B, the steadystate signal was 17.0 mV and the integrated decay was 633 mVs. The ratioof S/Σ for test gas B was 0.027 s⁻¹, which gives a relative diffusioncoefficient of 0.55. From the table above, test gas B is identified asbutane.

Cross sensitivity is defined as the signal produced per unit of gasconcentration of one gas, relative to standard gas. For combustiblegases, the concentration is often expressed as a percentage of the lowerexplosive limit. For the sensor 50 used in this example, the nominalcross sensitivities are as follows, relative to methane (=100%),Hydrogen=100% and Butane=65% (Product Data Handbook, vol. 1, Safety,issue 4.0, January 1997, City Technology Ltd., Portsmouth, UnitedKingdom).

Having identified test gas A as hydrogen and test gas B as butane, theconcentrations can be found from the steady state signal. The steadystate output of the sensor exposed to 40% methane in air was 47.7 mV.The steady state signal of the sensor exposed to test gas A was 50.2 mV,and since the cross sensitivity ratio for hydrogen is 100%, theconcentration of hydrogen in test gas A is estimated to be 40% LEL:(50.2/(1.0*47.7))=42% LEL hydrogen. Similarly, the steady state signalfor test gas B was 17.0 mV, and since test gas B was identified asbutane, the cross sensitivity ratio (relative to methane) is known to be65%. Thus, the concentration of butane in test gas B is estimated to be20% LEL: (17.0/(0.65*47.7)=22% LEL butane.

Test gas A was mixture of 50% LEL hydrogen in air produced by blending100% by volume hydrogen with zero air from compressed gas tanks using anelectronic mass flow controller, and test gas B was 0.473% by volume(25% LEL) butane in air from a compressed gas cylinder. The errorbetween the nominal test gas concentration and the measured value arethus 16% for test gas A and 12% for test gas B. This example has thusdemonstrated that the method described in this disclosure can be used toboth identify an unknown gas, and to estimate the concentration of thegas.

It is well known that gas diffusion coefficients vary with temperature Tand usually D is proportional to temperature raised to the power of 3/2(E. L. Cussler, Diffusion, Mass Transfer in Fluid Systems, CambridgeUniversity Press (1992)), and therefore temperature compensation will berequired. Since the response of most sensor types varies withtemperature, methods for providing temperature compensation are wellknown in the prior art pertaining to gas detection instruments.

The gas diffusion coefficient though usually written for a single gas isactually a function of the composition of all of the components of thegas mixture. Therefore, a diffusion coefficient calculated in one gasmixture may differ from that in another gas mixture. In general thisissue is not a problem, since most applications for gas detectioninstruments measure the analyte gas in air. If the air were replaced byanother gas, for example helium, then the diffusion coefficients of thegases will change. For example, the diffusion coefficient of anair-oxygen gas mixture is D=0.1775, but an air-helium gas mixture has adiffusion coefficient of 0.6242 cm² /s (E. L. Cussler, Diffusion, MassTransfer in Fluid Systems, Cambridge University Press (1992 )). If thisinvention is to be used in an application with a balance gas other thanair, then the change in the diffusion coefficients can easily becompensated for by calibrating the system with a known gas in a mixturewith the same balance gas as the application.

Obviously, while the valves 35 and 36 in FIG. 3 are closed, the sensoris not able to respond to changes in the gas concentration in theambient environment. One limitation of the above embodiment of theinvention is that the instrument is unable to monitor for changes in theambient atmosphere during the identification of the gas, since thesensor is exposed to the gas within the sealed compartment. Therefore,another embodiment of this invention uses two sensors, one of which isused to provide continuous monitoring and the other is used to identifythe gas. This system 60 is shown in FIG. 5. When valves 61 and 62 areopen, gas is pumped by pump 30 into the identification sensor manifold32, and the identification sensor 33 is exposed to the gas. When valves61 and 62 are closed, then the sensor 33 consumes all of the gas in thesensor manifold as described above to allow identification of the gas.Gas from pump 30 is also passed through tubing 63 to a second sensormanifold, 64 containing a continuous monitoring sensor 65, which makes agas tight seal with the manifold by means of O-ring seals 66. Afterpassing through the manifold 64, the gas flows via tubing 67 to theexhaust 37. The gas from pump 30 is always flowing through the tubing 63and manifold 64, and thus the sensor 65 provides continuous monitoring,even when valves 61 and 62 are closed, thus isolating sensor 33 fromchanges in the gas composition in the environment.

The time taken to wait for the response to decay to zero once the valves35 and 36 in FIG. 3, or valves 61 and 62 in FIG. 5 have been closed cantake a significant length of time, typically seconds to minutes. Thetime required will depend on the sensor 33, on the shape and volume ofthe chamber defined by the valves 35 and 36, or 61 and 62, and on thegas being identified. Furthermore, as the signal approaches zero, thesignal to noise ratio decreases and thus it is increasingly difficult toresolve the signal from the background signal, i.e. the signal presentwithout gas.

One way to avoid this problem is to measure the initial decay rate, andthen to extrapolate this curve to zero gas signal, by assuming amathematical function for the signal decay. Provided the gas in thechamber defined by the values 35 and 36, or 61 and 62, can readilydiffuse to the sensor 33, then the decay of the output signal may beapproximated by an exponential decay. Thus if the initial steady statesignal (S or I_(ss)) is measured, and the time taken for the signal todecrease to 50% of the initial value after closing the values 35 and 36,or 61 and 62 is measured, then the integrated signal under the curve (Σor Q) can be calculated.

Using a catalytic bead sensor as an example, and assuming an exponentialdecay, the output is described by the following equation:

    S.sub.t =S.exp(-bt)

in which S_(t) is the output signal at time t, S is the initial steadystate signal prior to closing valves 35 and 36, or 61 and 62 and b is aconstant. When the output response has fallen to 50% of the steady statevalue, the expression can be re-written as follows:

    S.sub.t /S=0.5=exp(-bt.sub.50)

in which t₅₀ is the time taken for the signal to decrease to 50% of theinitial value from the initial time (t=0) when the valves 35 and 36, or61 and 62 were closed. Rearranging this equation for b gives thefollowing result:

    b=0.693/t.sub.50

Integration of the function S_(t) =S.exp(-bt) with respect to time,gives the cumulative signal Σ:

    Σ=[-S/b(exp (-bt))]

Evaluating this function from the times that the valves 35 and 36, or 61and 62 were closed (t=0) to long times (t=>∞), gives the result

    Σ=S/b

Since the steady state current S is known, and the constant b can bemeasured, the integrated response can be found more quickly than havingto wait for the response to decay to zero.

This method of extrapolating the time taken for an exponential decay iswell known in the prior art, and it is included here to illustrate theprinciple. Clearly, many other methods of extrapolating the decay arepossible, and are well known to those experienced in the art. Forexample, Tantram et al in U.S. Pat. No. 4,829,809 disclose a method formeasuring the concentration of a gas by exposing an electrochemicalsensor to a known volume of the gas to be measured. The output signalfrom the sensor decays as the gas is consumed, and by measuring thetotal charged passed, and knowing how many electrons are passed for theoxidation or reduction of the gas, they can use Faraday's law tocalculate the concentration of the gas.

    C=Q/nFV

Rather than wait for the sensor response to decay all the way to zero,Tantram et al propose a method for speeding up the measurement, byassuming an exponential decay, and extrapolating to zero current. Theapparatus described by Tantram et al is similar to that described inFIG. 3 of this application. Thus, once the gas had been identified usingthe method described in this disclosure, it is possible to measure theconcentration of the gas using the method of Tantram et al. For acatalytic bead sensor, a similar relationship exists, and thus theconcentration of a combustible gas sensor can also be found.

    C=Σ/KHV

However, whereas Faraday's constant is a physical constant available inany physical chemistry data book, the constant K for the catalytic beadsensor will vary from sensor to sensor, and H is a gas specificconstant. Once the gas has been identified, then the gas specificconstant H can be used, if previously determined. For most combustiblegases, H is approximately equal to one, if the response is calculated interms of the LEL. The constant K is most conveniently determined bycalibrating the gas detection instrument with a gas of known identityand of known gas concentration.

In a further embodiment of this invention, if the identity of the testgas is known, and the sensor response is limited by diffusion, then ifthe calculated gas diffusion coefficient is close to the expected value,then the sensor is deemed to be operating under diffusion control.However, if the measured diffusion coefficient differs greatly from theexpected value of the diffusion coefficient, then there is a problemwith either the instrument, the gas delivery system (e.g. blockedtubing), or with the sensor. Thus this method can also be used toascertain the functional status of the sensor and instrument. While thisinvention has been described for electrochemical and catalytic beadsensors, it is readily apparent that the same invention may be appliedto any kind of sensor whose response is limited by the diffusion rate ofthe analyte species. Moreover, it will be evident to those skilled inthe art that numerous variations of the details may be made withoutdeparting from the invention as defined in the appended claims.

What is claimed is:
 1. Method for identifying a reactive gas in acarrier gas, comprising the steps of:obtaining a detector whichgenerates an electrical signal upon exposure to the gas; positioningsaid detector to accept gas from a manifold of defined volume with adiffusion barrier between the detector and the manifold; determining anoutput signal from the detector for the carrier gas without the reactivegas; directing the reactive gas and carrier gas through an inlet intothe manifold, obtaining a steady state signal S from the detectorindicating presence of the reactive gas and exhausting carrier gas andreactive gas from the manifold through an outlet; closing the inlet andoutlet, and permitting reactive gas in the manifold to diffuse throughthe diffusion barrier for analysis, determining a decay profile as thesteady state signal is being reduced vs. time to the output signal forthe carrier gas as reactive gas is consumed; determining from the steadystate signal and decay profile a diffusion coefficient for the reactivegas, and identifying the reactive gas from the diffusion coefficient. 2.The method of claim 1, wherein the diffusion coefficient is determinedby integrating the decay profile to obtain an integrated response Σ,calculating a relative diffusion coefficient S/Σ, comparing the relativediffusion coefficient with a relative diffusion coefficient for a knowngas and identifying the reactive gas from the comparison with the knowngas.
 3. The method of claim 2, additionally comprising determining theconcentration of reactive gas in the carrier gas by comparing the steadystate signal for the reactive gas with a steady state signal for theknown gas.
 4. The method of claim 2, wherein the known gas is methane.5. The method of claim 1, wherein the detector is a catalytic beaddetector.
 6. The method of claim 1, wherein the detector is anelectrochemical detector.
 7. The method of claim 1, wherein the reactivegas is selected from the group consisting of combustible gases and toxicgases.
 8. The method of claim 7, wherein the reactive gas is acombustible gas selected from the group consisting of hydrogen, alkanes,alkenes, alkynes, alcohols, ethers, esters and aromatics.
 9. The methodof claim 8, wherein the reactive gas is selected from the groupconsisting of methane, hydrogen, butane and pentane.
 10. The method ofclaim 7, wherein the reactive gas is a toxic gas selected from the groupconsisting of carbon monoxide, sulfur dioxide, hydrogen sulfide, nitricoxide, nitrogen dioxide and chlorine.